Hydrous oxide growth

Thick hydrous oxide films on gold can readily be produced, as in the case of platinum, by either dc or potential cycling techniques. 

The conditions required to produce a thick hydrous oxide layer on iridium under potential cycling conditions were investigated 

Two fundamental properties influencing the electrochemical behavior of acid-growth hydrous iridium oxide films are (1) the acidity of the hydrated material and the variation of this property with change in the iridium oxidation state,4 and (2) the difference in electrical conductivity between the reduced and oxidized form of the surface film.170 174 According to Burke et al.9M both properties 

The film was produced by repetitive potential cycling in the acid medium (from Ref. 186, with permission). 

An account of a comparison of the potential/pH behavior of hydrous and anhydrous iridium oxide films was published recently.145 The open-circuit response for the hydrous material, in the half charged state, was ca. 1.25(2.3RT/F) V/pH unit this lower 

Polarizing the metal in acid above ca. 1.80 V results in the formation of a reddish brown oxide layer which on reduction yields a relatively active form of the finely divided metal, i.e., a type of gold black. The reactions involved were investigated using potential sweep techniques by Lohrengel and Schulze, who found that rapid growth in add required a potential in excess of 2.0 V a multilayer film was clearly formed under such conditions as two, and occasionally three, peaks were recorded on the cathodic sweep. These authors postulated that while the inner layer was both a poor electronic and ionic conductor, the reverse conditions prevailed for the thicker outer film, which they also suggested was rather compact in character. 

In a recent ESCA study of films produced by potential cycling it was found that the initial layer (CEF =1) was not I1O2 but the hydrated hydroxide—some water was assumed to be chemisorbed on the surface beneath the Ir(OH)4 species. With thicker films (CEF = 80) some peaks attributable to Ir(VI) were also evident appreciable incorporation of S04 anions from solution was observed. It was not possible to examine the reduced state as the dried film was rather readily oxidized in air. Electron microscopy combined with X-ray emission analysis showed that surprising differences in morphology arise in oxide deposits generated electrochemically on iridium and ruthenium. In the case of iridium the oxide was classified as an amorphous, hyperextended material. X-ray emission analysis showed a much higher 0/Me ratio in the oxide formed on iridium as compared with ruthenium. 

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The mechanism of hydrous oxide growth on repetitive cycling is now reasonably well understood, at least at a qualitative level. 

Figure 1. Schematic outline of the processes involved in hydrous oxide growth under potential cycling conditions.

Work of a similar nature, involving hydrous oxide growth on platinum under square-wave perturbation conditions in acid, has been reported recently by Chialvo et al.137 Changes in real surface area were monitored by measuring the hydrogen monolayer charge before and after the hydrous oxide growth and reduction processes. The optimum limits observed in this case (especially the lower value... 

There are a number of reasons as to why hydrous oxide growth is not observed on cycling in strong base. Local pH changes, and therefore hydrous rearrangement, would not occur in these more highly buffered solutions. Conway and Mozota30 attributed the lack... 

This system seems to be the only alloy to date whose hydrous oxide growth behavior under potential cycling conditions has been investigated.189 190 Burke and O Sullivan189 demonstrated that with an alloy containing 10% by weight of rhodium in platinum both components corroded on cycling between certain limits (0-1.5 V) in 1.0 mol dm-3 NaOH. However, while the platinum corrosion product was found to be soluble under these conditions the rhodium one was not—in fact the hydrous film developed on the surface in this case was apparently derived almost totally from the minor component in the alloy. 

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